Extendible stepladders are needed for use on stairway, on any inclined surface, for instance, a sloping driveway. It is also necessary to provide a safe stepladder so that the workman will not be exposed to falls and serious injury.
Many proposals of self-leveling ladders have been made. U.S. Pat. No. 2,481,581 which issued in 1949 describes a device which may be applied to either the front or back legs to vary the length of any one of the four legs. According to this patent, in order to extend or lengthen the front leg structure at the lower end of each of the front legs or sides, a guide channel is provided which is long enough to constitute a sleeve. This sleeve is shaped in cross section to present a side portion and edge flanges in opposed relation to firmly fit the side and the edges of the legs. The channels are held in place by screws to provide a passage through which is mounted for sliding movement an extension member so that it can be manually moved up and down for adjustment beyond the end of the leg to extend the ladder structure at the front. The sleeve has fastened an internally threaded nut which is aligned with a similar opening in the sleeve to receive a threaded end of an adjusting handle. The handle is manipulated to jam the extension piece in a desired position. Similar sleeves and extension members are provided for the rear legs. The drawback with this extendible stepladder is that it is not sufficiently stable because the extension members are only held on one side of the legs of the ladder.
Another proposal has been made in U.S. Pat. No. 4,676,342 which describes a tressle with rails consisting of a hollow tube within which is disposed a telescoping extension. The rails have openings and there are bolts and spring plates for securing the extension member to the leg. The possibility that the extension member will slip out with substantial injury to the workman also exists because only one lock system is used. Also the legs of this tressle are very thin and the disc used is not sufficiently firm.